PC boots up (bar is 1/2 way)turns off, repeats intermittently then dies doesn't reach the XP logo screen
often a driver corrupted.
Also make sure nothing plugged ito usb firewire etc.
remove printers scanners etc.
PC will turn on after being switched off
PSU faulty or Bios set to wake on LAN or wake on ring
PC completely dead no power on (peripherals ok ie kb/monitor/ex HDD,DVD etc)
PSU or mobo failure